Refrigeration systems, as for air conditioning in large buildings, typically employ multiple compressors in either parallel or series flow for chilling of a controlled medium such as process water, cold plenum, chill water or return water. In many such systems the chilling capacity of the compressor may be varied. Some systems do not require the chilling capacity of a multiple compressor system, and use only a single, variable capacity compressor. Each such compressor, for example the York Model HT 250 with electrical control, typically has internal controls and safety devices which provide for failure shutdown and overload protections. However, such internal controls are effective only in controlling the individual compressor and thus have limited value in a multiple compressor system. Further, the internal controls do not react automatically to changes in the demand for chilling capacity, and are thus of limited value in even single compressor systems.
Because of the substantial power demands of such refrigeration systems, especially in view of the rising cost of energy, and the inconvenience resulting from system failures, it is important not only that a compressor system be kept operating, but that the system operate efficiently. Toward this end, various forms of control devices have been employed to turn on or turn off and increase or decrease chilling capacities of the individual variable capacity compressors within the system. For screw-type compressors, capacity is varied by the relative position of the coolant inlet nozzle. For centrifugal compressors, capacity is varied by varying vane position. However, capacity need not in all cases vary directly with vane position. Nevertheless, for purposes of simplicity capacity and vane position are used herein as being substantially interchangeable.
Certain of these prior art control devices have suffered from overloading proglems on initial startup, raising the possibility of nuisance failures--i.e., a shutdown of the system when actually no system failure exists. For example, on initial startup many control devices permit the system compressors to increase to maximum capacity over a short time period, thereby causing such nuisance failures as low refrigerant temperature shutdown, or overshoot on water temperature control, when actually no system failure exists. Such a rapid increase in capacity also results in a large short term increase in power consumption. A similar problem occurs when a first compressor is at maximum capacity and demand requires the additional chilling capacity of another system compressor, resulting in increased power consumption for an extended period of time on startup of the second compressor.
One solution to this problem used with reciprocal compressors has been to periodically sense a thermostat and to permit the capacity of the compressor to vary by only limited amounts each time the thermostat is sensed. Other prior art control devices have attempted to avoid these sorts of nuisance failures by a circuit commonly referred to as a "load limiter", which essentially senses the load current of the compressor and compares that current to a known maximum value. The control device then allows only a percentage increase in load current during a given period.
The difficulty with the load limiter type of control device is especially apparent in variable capacity centrifugal compressors, in either multiple or single compressor systems. In centrifugal compressors load current increases are not necessarily indicative of variations in compressor capacity. For example, when a small amount of material of a viscosity different from the state of the normal refrigerant is accidentally drawn into the compressor, load current will increase without varying the capacity. This can cause a current "spike" in the load limiter circuit, which in turn causes the load limiter control to improperly reduce capacity.
Another problem found in conventional control devices has been a difficulty in maintaining a balance in capacities of the system compressors. Most multiple compressor systems employ compressors of equal capacity. To maximize the operating efficiency of such a system, thereby minimizing power demand, it is desirable to equalize, or balance, the capacity of each machine at any given moment during operation. For systems employing compressors of differnt capacities, a weighted balance is desirable.
This balancing requires two steps: first, that the capacities of the operating compressors be initially balanced; and, second, that increases or decreases in capacity be distributed between the operating compressors so as to substantially maintain a balance in chilling capacities. The distribution of increases or decreases may be accomplished either by signalling both compressors to load or unload a given amount, or by alternating the loading (or unloading) of the compressors. Typically, the latter option is chosen to minimize enery demand; however, accurate sensing of compressor capacity, coupled with an efficient alternating control has proven difficult.
In single compressor systems, accurate control has also proven difficult. In addition to the problems previously discussed, most prior art control systems can provide only a few incremental changes in chilling capacity, such that gross changes in demand are required before the control device will respond with a variation in capacity.